Question about Codes and a weird "Check engine light stays on even when car is off" issue

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Question about Codes and a weird "Check engine light stays on even when car is off" issue

All,
Firstly this is for my (Gerry's and Dave's old) 1995 Mercedes E320 Wagon. It has the 16 pin connector, and I have the little blink box code reader (two of them actually, as I keep one in each car). I had the check engine light come on the other day (right after it rained in CA) and I thought "meh, it is some silly code". So I scanned the codes, and "cleared" them (more on that in a sec) and the light went out. And then one drive cycle later, the light came back on (the red, CEL on the dash all the way to the right) and STAYED ON no matter what I did (even with the key out of the car).

I poked around a bit, and saw some stuff about how it might be the Overload protection relay. So I disconnected the battery, and took a look at that little sucker.

It had some moisture under the plastic flip top "cap" over the 15 A fuse. I took it out, and cleaned it all out with contact cleaner, and let it dry. Then I put the Overload Relay back, and I ziptied a plastic baggie over it in case it was getting moisture from some place.

Everything was good for a bit, and then the lights came back....

Note that the car doesn't seem to have ANY issues. It runs and drives fine, and doesn't seem to have any problems that I can notice.

Now, I was able to easily clear the Code 6 on Pin 14, but the others didn't seem to want to clear. Is there some trick to clearing them? The instructions say read the code, and then press the button for 8-10 seconds and it should clear. But when I re-read the codes, they seem to not be cleared.

Ideas? I ordered a new Overload Protection Relay, but it won't be here for a bit. It also weirds me out that a check engine light stays on ALL the time.

The Following User Says Thank You to Klink For This Useful Post:

Re: Question about Codes and a weird "Check engine light stays on even when car is off" issue

Dan, the instructions I've seen say to wait 3 seconds after reading the code, then hold 6-8 seconds to clear. I'm not sure how critical the timing is, and since you were able to clear some codes, this may be unrelated.

For the SRS codes, if the SRS lamp is not on while driving, I wouldn't lose sleep over it. (I also can't remember if I ever checked codes on pin 6!) Note that the full description for code #9 is "SRS malfunction indicator lamp (A1e15) or time limit for DTC readout /erasing exceeded" so also make sure the SRS bulb is good - it should turn on for ~4 seconds or so when all the other dash lights turn on at ignition position 2. If you don't have my M104 DTC document, click here, or see attached.

On to the DM (pin 3). The DM codes often will not clear until the engine is started after clearing DM codes. Also, the one time I've seen the CEL remain on with the ignition off, the root cause was a faulty / defective DM. Replacing the DM cured the problem on that car (an 036). The DM is a rare failure and you should be able to locate a used one quite cheap if needed. You can also just disconnect the silly thing for now, the DM serves no purpose except as a watchdog for the other modules. When it sees anything it doesn't like, it barks out a CEL on the dash to scare the driver into bringing the car to the dealer for service.

PS: The DM (N59) is located in the passenger footwell on the V8 models (.034/.036). On the 6-cyl cars, it is below the instrument cluster, to the left of the steering column. Screen shot attached from FSM. TO disconnect / remove you twist the round black knob.

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Re: Question about Codes and a weird "Check engine light stays on even when car is off" issue

Dave: Its Peter Weissman here. It was my understanding on our W124 M104 cars the CEL was related to emissions issues and would not illuminate due to the codes mentioned earlier. You didn't mention a bad EGR valve and/or a clogged EGR tube as possible causes for the CEL to be on. Shouldn't these be checked out.
Also based on my experience a bad OVP relay leads to a no start situation which isn't the case here.

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Re: Question about Codes and a weird "Check engine light stays on even when car is off" issue

Originally Posted by geraniumtr

Dave: Its Peter Weissman here. It was my understanding on our W124 M104 cars the CEL was related to emissions issues and would not illuminate due to the codes mentioned earlier. You didn't mention a bad EGR valve and/or a clogged EGR tube as possible causes for the CEL to be on. Shouldn't these be checked out.
Also based on my experience a bad OVP relay leads to a no start situation which isn't the case here.

Hi Peter,

You are correct, the DM primarily is watching for any faults which could affect emissions, and turns on the CEL when certain criteria are met. It's possible to have a code on the DM without the CEL illuminated. The only code Dan mentioned which might trigger the CEL would be code 17 (CAN data) on pin 3 (DM). The DM does not monitor the SRS module at all, and I don't think it monitors the E-GAS (pin 14) either but I'm not 100% certain. The major concern here is the CEL remaining illuminated with the ignition off and key removed, which likely indicates a defective/failed DM.

If there were EGR or other issues, that should have triggered a separate code on pin #3, assuming the DM is working correctly. Any time the CEL is on, the codes on the DM will indicate why. In this case it's complaining about the CAN, which might possibly be related to the DM itself failing, since the car is driving normally. Ditto for the OVP, if there were OVP issues there should be other problems besides the CEL.

Klink, have you ever seen a good DM illuminate the CEL with the ignition off / key removed? If so, do you remember what fixed it?